Semiconductor light emitting elements typified by light emitting diodes (LED) and organic EL (OLED) have been required to have high luminance as a light source to be used for illumination. A method for improving the light extraction efficiency by using a sapphire substrate that has micron-size projections/recesses formed on its surface (PSS: Patterned Sapphire Substrate) is common. Further, as a new method for improving the light extraction efficiency, a technique of forming a photonic crystal periodic structure with a period that is about equal to the wavelength of light on a light extraction layer has been introduced. Typically, a photonic crystal periodic structure is formed at the interface between two structures with different refractive indices, and has projections/recesses that are mainly formed of pillar structures or hole structures. It is known that in a region in which such a periodic structure is formed, the presence of light is prohibited and total reflection is thus suppressed; thus, using this can contribute to improving the light extraction efficiency.
However, although light emitted from a light emitting layer of a semiconductor light emitting element includes both transversal magnetic light (TM light) and transversal electric light (TE light), optimization of the behavior of each light in the periodic structure, that is, optimization of the periodic structure depending on whether the periodic structure reflects or transmits each light has not been clarified.
It is known that light emitted from an InGaN-based LED is mainly TE light, while light emitted from an AlxGa1-xN-based LED, which is obtained by forming a single well layer on the c-plane of a sapphire substrate, is mainly TM light if the content of Al is in the range of up to x=0.25, and the proportion of TM light will increase with an increase in x (it should be noted that the properties of TE/TM polarized light in the quantum well were evaluated by measuring the intensity of photoluminescence at a temperature of 8.5 K “Applied Physics Letters,” No. 25, Jun. 21, 2004, Vol. 84).
Further, it has been reported that when an AlN buffer layer and a multi-quantum well of AlxGa1-xN/AlN are grown on the c-plane of a sapphire substrate, TM light becomes dominant if the content of Al is in the range of up to x=0.82, and the proportion TM light will increase if the thickness of the well layer is set to less than or equal to 3 nm with the content of Al unchanged (“Physical Review,” B 79, 121308(2009)”).
The relationship between TE light and TM light in the present invention is defined as follows. In the present invention, TE light refers to light whose electric field components oscillate in the transverse direction with respect to the plane of a photonic crystal periodic structure, that is, a plane along a direction in which the periodic structure changes periodically, while TM light refers to light whose magnetic field components oscillate in the transverse direction with respect to the plane of a periodic structure. That is, TM light is light whose electric field components oscillate in the direction perpendicular to the plane of the periodic structure.
In Patent Literature 1, when the parameters of a photonic crystal periodic structure in a light extraction layer are designed, a photonic band structure for each of TE light and TM light is computed, and consequently, TE light for which a wider PBG exists is selected to optimize the design of the parameters of the periodic structure.
In Patent Literature 2, a waveguide is designed in a photonic crystal slab with a PBG for each of a TM-like mode and a TE-like mode, whereby leakage of light into the crystal plane is prevented, and a high Q value is thus exhibited.
In Patent Literature 3, a first conductive type semiconductor layer has formed thereon a photonic crystal periodic crystal structure that contains SiO2 and air gaps and has a smaller refractive index than the first conductive type semiconductor layer, whereby light is allowed to become incident on a non-conductive semiconductor layer thereon.
In Patent Literature 4, photonic crystals are formed between a first phosphor layer and a second phosphor layer so as to transmit second converted light and reflect first converted light.
Patent Literature 5 discloses a structure in which a fluorescent material layer that has a two-dimensional periodic structure and has a photonic band gap for TE polarized light is provided on an outer portion of an optical waveguide, thus providing the effect of efficiently confining TE polarized light in the optical waveguide.